1. Field of the Invention
The present invention relates to a vane air motor usable as driving means for air tools such as pneumatic grinders.
2. Description of the Related Art
A vane air motor has a rotor housing and a vaned rotor. The rotor housing comprises a cylindrical wall having a circular cylindrical inner peripheral surface defining a rotor chamber and end walls provided to close the opposite ends of the cylindrical wall. The vaned rotor is rotatably disposed in the rotor chamber eccentrically with respect to the latter. Compressed air is supplied into the rotor chamber from an air supply opening provided in the cylindrical inner peripheral surface, and the vaned rotor is rotationally driven by the compressed air. The compressed air that has finished rotationally driving of the rotor is discharged to the outside of the rotor chamber from an air discharge opening that opens on the cylindrical inner peripheral surface (Patent Literature 1 noted below).
The rotor has an output shaft portion projecting from one end surface of the rotor along the axis of rotation of the rotor and rotatably supported by one end wall of the motor housing and a support shaft portion projecting from the other end surface of the rotor in coaxial relation to the output shaft portion and rotatably supported by the other end wall of the motor housing. The output shaft portion is drivably connected to a member performing a desired tool function, e.g. polishing, of a pneumatic grinder or other tool concerned. On the other hand, the support shaft portion is, usually, connected to a governor that limits, when the rotor is rotated at a number of revolutions greater than a predetermined one, an air supply flow path for supplying compressed air to an intake hole communicating with the rotor chamber, thereby suppressing the number of revolutions of the rotor. The motor housing and the governor are enclosed by a casing of a pneumatic grinder or other tool to which the vane air motor is attached, and compressed air to be supplied into the rotor chamber is supplied through a compressed air supply chamber formed around the governor by the casing and through the end wall of the motor housing (Patent Literature 2 noted below).
Citation List: Patent Literature:
    Patent Literature 1: Japanese Patent Application Publication No. Sho 56-34905    Patent Literature 2: Japanese Patent Application Publication No. 2001-9695
The vanes are each formed in a thin-plate shape, and in response to the rotation of the rotor, the vanes are displaced radially of the rotor and rotate while maintaining sliding engagement with the cylindrical wall surface of the rotor chamber. Therefore, the vanes are subjected to friction, impact associated with displacement, bending stress, and so forth, and hence difficult to use over a long period of time. Accordingly, it is desired to improve the durability of the vanes. However, it is difficult to clarify causes of impairing the vane durability because the vanes are rotated at high speed in the closed rotor chamber, and there has been no satisfactory improvement in durability. The inventor of this application wrestled with this problem and found the following causes of impairing durability.
The first cause is wear of the vane distal edge sliding on the cylindrical wall surface of the rotor chamber. The inventor of this application investigated the matter and found that the wear of the vane distal edge has an effect on the durability of the vane concerned even if the wear is not so large in scale that it is visually discernible. That is, regarding sliding of the vane distal edge on the cylindrical inner peripheral surface of the rotor chamber, because the inner peripheral surface is provided with the air supply opening and the air discharge opening, portions of the vane distal edge that pass across the air supply and discharge openings are less subjected to friction than the rest of the vane distal edge by an amount corresponding to the distance that the above-described portions travel to cross the respective openings, and therefore less worn than the rest of the vane distal edge. The air supply and discharge openings are spaced from each other in the axial direction of the rotor chamber. Therefore, a wear difference occurs between portions of the vane distal edge passing across the respective openings and portions thereof not passing across either of the openings, resulting in the vane distal edge being unevenly worn. In other words, the portions of the vane distal edge passing across the openings become projected, although only slightly, radially outward more than the rest of the vane distal edge, which does not pass across either of the openings. Because the vanes are rotated at high speed, the projecting portions of the vane distal edge hit the edges of the openings, causing large impacts. This interferes with the smooth rotation of the rotor and gives impact to the vane concerned, causing breakage of the vane. Further, the inventor of this application found that the uneven wear of the vane distal edge is mainly caused by the air discharge opening. That is, at a circumferential position where the air supply opening is present, the vane is pressed radially inward by compressed air supplied through the opening, and therefore, the friction between the vane distal edge and the wall surface of the rotor chamber is reduced, whereas, at a circumferential position where the air discharge opening is present, compressed air is discharged from the air discharge opening, and therefore, much larger friction is produced between the vane distal edge and the rotor chamber wall surface than at the position where the air supply opening is present. Consequently, the above-described wear occurs.
In regard to the durability of the vanes, the inventor of this application also noticed the following point: A conventional vane air motor is arranged as follows. Regarding compressed air supplied through an intake hole provided in one end wall of the rotor chamber, a part of the compressed air is supplied into the rotor chamber through air supply openings provided in one end portion of the above-described cylindrical wall that is adjacent to the end wall. The rest of the compressed air is passed through an intake passage extending through the cylindrical wall in the axial direction thereof as far as the other end of the cylindrical wall, and supplied into the rotor chamber through the other air supply openings provided in the other end portion of the cylindrical wall. In such a type of vane air motor, breakage is likely to occur at the above-noted one end portion of the vane distal edge. The inventor of this application found that the cause of the breakage is due to the following matter: In the vane air motor having the above-described structure, a difference in pressure is likely generated between the flows of the compressed air supplied into the rotor chamber from the air supply openings in the one end portion and the other end portion of the cylindrical wall. Accordingly, the opposite ends of the vane are subject to the flows of the compressed air supplied in radially inward from those openings under different pressures. Consequently, the vane is rotated together with the rotor with the distal edge thereof inclined, and the one end portion of the vane distal edge is pressed against the cylindrical wall surface with a stronger force than the other end portion thereof. For this reason, the one end portion of the vane distal edge is likely to become worn. When passing across the above-described air supply openings, in particular, the one end portion of the vane distal edge that is pressed against the cylindrical wall surface hits the peripheral edges of the openings and receives a large impact, resulting in a rupture at the one end portion of the vane distal edge. It is also deemed that the impact applied to the one end portion of the vane distal edge has an effect on the whole vane and causes a rupture at a portion of the vane distal edge other than the end portion thereof.
Further, the inventor of this application found that the following is the reason why wear or breakage is likely to occur at the one end portion of the vane distal edge. The output shaft portion and support shaft portion of the rotor are supported by the respective radial bearings. The radial bearing supporting the support shaft portion is adjacent to the above-described compressed air supply chamber. Therefore, the pressure of compressed air acts on one side (side remote from the rotor chamber) of the radial bearing, causing grease in the radial bearing to leak into the end portion of the rotor chamber. Because grease has a high viscosity, if the grease entering the rotor chamber adheres to the corresponding end portion of a rotating blade, the grease hinders smooth radial movement of the blade relative to the rotor. This may also cause the blade to be inclined and give rise to a problem similar to the above.
Further, the inventor of this application noticed the following: The vanes are each formed in the shape of an elongated plate that is long in the axial direction of the rotor and that has a short width in the radial direction of the rotor. In this regard, the inventor noticed that an axially extending rupture may occur in a vane at a substantially middle position in the width direction, and found that the cause of the rupture is as follows: Each vane is accommodated in a radially extending groove provided on the rotor so as to move radially outward and inward within the groove in response to the rotation of the rotor. Therefore, the side surfaces of the vane slide on the side walls of the groove. In addition, the distal edge of the vane slides on the cylindrical inner peripheral surface of the rotor chamber and therefore encounters resistance to rotation from the cylindrical inner peripheral surface. Consequently, the vane moves outward and inward within the groove while being rotated with a slight inclination in the direction of rotation. Accordingly, a side surface of the vane receives friction from contact with the side wall and edge of the groove, resulting in the vane side surface being scraped, although only slightly. Such a scraped portion of the vane side surface is weak in mechanical strength and readily crackable because the vane is rotated at high speed and subjected to a large impact as stated above. Eventually, a rupture will occur in the scraped portion of the vane side surface.
The inventor of this application found that the above-described causes relate to the durability of the vanes, and that the causes interact with each other to impair the durability of the vanes.